1. Field of the Invention
This invention relates to a thermal expansion valve and, more particularly, to a thermal expansion valve combined with a thermal bulb.
2. Description of the Related Art
A thermal expansion valve is used together with a compressor, a condenser and an evaporator in a refrigeration apparatus using a refrigerant, and controls the flow rate of the refrigerant flowing into the evaporator in response to the temperature of the refrigerant at an outlet port of the evaporator.
A typical thermal expansion valve comprises: a thermal bulb in which a heat sensitive working fluid is sealed and which is located at the outlet of the evaporator and produces a pressure of a gas of the working fluid in response to the temperature of the refrigerant at the outlet port of the evaporator; a power element which has a diaphragm, communicates with the thermal bulb by a capillary tube and activates the diaphragm in response to the pressure of the gas of the working fluid in the thermal bulb; and, a valve housing which is adjacent to and combined with the power element, in which two independent refrigerant flow passages are provided, and which holds a valve body to move relative to a valve seat formed in one refrigerant flow passage and also holds a valve body drive member for transmitting a deflection of the diaphragm of the power element to the valve body to make it sit on and separate from the valve seat in response to the deflection of the diaphragm (that is, the temperature of the refrigerant at the outlet port of the evaporator).
When the conventional thermal expansion valve having such a configuration as described above is used for an air conditioner of an automobile, particularly for a compact car, it is troublesome to install the long and fine capillary tube in a small engine room and the capillary tube is liable to be damaged under a maintenance and repair work in the engine room.
For these reasons, the power element and the thermal bulb are combined with each other in a thermal expansion valve of an automobile air conditioner and does not use any capillary tube. FIG. 3 shows a longitudinal sectional view of the conventional thermal expansion valve of the automobile air conditioner.
In a valve housing 10 of the thermal expansion valve, a first refrigerant passage 14 and a second refrigerant passage 16 are formed independently from each other, and a valve seat 12 is formed in the first refrigerant passage 14. One end of the first refrigerant passage 14 is connected to an inlet port of an evaporator, an outlet port of the evaporator is connected to the other end of the first refrigerand passage 14 by way of the second refrigerant passage 16, an compressor, a condenser and a reservoir.
A valve body 18 is disposed in the first refrigerant passage 14 and is urged to sit on the valve seat 12 by urging means 17. A power element 20 having a diaphragm 19 is fixed to the valve housing 10 and is disposed adjacent to the second refrigerant passage 16. One chamber 20a partitioned by the diaphragm 19 in the power element 20 is airtightly sealed and contains a heat sensitive working fluid used in a conventional thermal bulb.
A short capillary tube 21 extending from the sealed chamber 20a of the power element 20 is used to degas from or inject the heat sensitive working fluid into the chamber 20a and the extended end of the tube 21 is airtightly sealed after the completion of degassing and injection.
In another chamber 20b of the power element 20, an extended end of a valve body drive member 22 extending from the valve body 18 through the second refrigerant passage 16 in the valve housing 10 is disposed and abuts the diaphragm 19. The valve body drive member 22 is made of a material having a large heat capacity and transmits heat of a vapor of the refrigerant, flowing out from the outlet port of the evaporator and flowing into the second refrigerant passage 16, to the heat sensitive working fluid in the sealed chamber 20a of the power element 20, so that the working fluid provides a working gas having a pressure in response to a temperature of the vapor of the refrigerant. The other chamber 20bcommunicates with the second refrigerant passage 16 within the valve housing 10 by way of a peripheral gap of the valve body drive member 22.
Thus, under the influence of the urging force of the urging means 17, the diaphragm 19 of the power element 20 controls the degree of the opening of the valve body 18 relative to the valve seat 12 (that is, the flow rate of the liquid refrigerant flowing into the inlet port of the evaporator) in response to the difference between the pressure of the gas of the heat sensitive working fluid in the sealed chamber 20a of the power element 20 and that of the refrigerant vapor in the other chamber 20b or in the outlet port of the evaporator (it is considered that the pressure difference is in proportion to the degree of superheat defined by a difference between the temperature of the refrigerant vapor at the outlet port of the evaporator and that of evaporation of the refrigerant in the evaporator).
This conventional combined type thermal expansion valve can be easily installed in the air conditioner of the automobile, particularly the compact car. But, since the sealed chamber 20a of the power element 20 projects into the space of the engine room, the heat sensitive working fluid in the sealed chamber 20a is influenced by not only the temperature of the refrigerant vapor at the outlet port of the evaporator, transmitted through the valve body drive member 22, but also the temperature of the atmosphere in the engine room.
Therefore, the thermal expansion valve influenced by the atmosphere can not fully work its function.
FIG. 4 shows a thermal expansion valve proposed in U.S. Pat. No. 3,537,645 and improved to eliminate the above disadvantages of the above described conventional thermal expansion valve.
The same components of the improved thermal expansion valve of FIG. 4 as those of the valve of FIG. 3 are indicated by the same reference numerals as those of FIG. 3 and their detailed description will be omitted here.
In the improved conventional thermal expansion valve, an end portion of the valve body drive member 22 located adjacent to the diaphragm 19 is inserted into an opening formed in a center of the diaphragm 19 and is firmly fixed to the central opening of the diaphragm 19. A blind hole 22a is bored in an end surface of the end portion of the valve body drive member 22 to open to the sealed chamber 20a of the power element 20. Since the heat sensitive working fluid in the sealed chamber 20a of the power element 20 can flow into and flow out from the blind hole 22a of the valve body drive member 22, the working fluid functions in greatly response to the temperature of the refrigerant vapor at the outlet port of the evaporator than in response to the temperature of the atmosphere in the engine room.
The improved conventional thermal expansion valve, however, is too sensitive to and excessively respond to the temperature of the refrigerant vapor at the outlet port of the evaporator, so that it makes the valve body 18 frequently move between opening and closed positions (a "hunting" phenomenon). Such a phenomenon makes the performance of the air conditioner be unstable and significantly reduces its efficiency.
Further, in the improved conventional thermal expansion valve, an airtight sealing at the fixing between the central opening of the diaphragm 19 and the corresponding end of the valve body drive member 2 and consequent reduction in the durability of the diaphragm 19 are in trouble.
FIG. 5 shows an enlarged view of the fixing between the central opening of the diaphragm 19 and the corresponding end of the valve body drive member 22. A step is formed in the outer peripheral surface of the end portion of the valve body drive member 22. A diaphragm support member 22b is stacked on the step, and the peripheral portion of the central opening of the diaphragm 19 and a diaphragm catch 22c are placed successively on the diaphragm support member 22b, and an airtight of the central opening of the diaphragm 19 is produced by welding a peripheral edge 22d of the diaphragm catch 22c to the surface of the diaphragm 19.
If the welding is carried out sufficiently to ensure the airtight, the inner peripheral edge of the thin diaphragm 19 surrounding the central opening tends to become brittle by heat due to the welding. Consequently, the inner peripheral edge of the diaphragm 19 surrounding the central opening is fatigued and is broken easily after a relatively small number of its deflection.
The improved conventional thermal expansion valve as described above is, therefore, still defective in terms of durability and such thermal expansion valves are not actually used.